Nuclear fusion reactors hold promise to supplant the need for increasingly scarce fossil fuel energy resources. Fusion power with its low generation of radioactive waste is believed to be an ideal candidate for large-scale energy generation. Although many fusion reactions have been proposed, leading fuels considered for fusion power generation are isotopes of hydrogen, namely deuterium and tritium. Other reactions involving helium3 are also under consideration. A leading fusion reaction for practical energy generation, referred to as the D-T fusion reaction, involves fusing deuterium and tritium atoms to produce helium and high energy neutrons. However, due to its short half-life tritium is difficult to find, and expensive. The D-T fusion reactors typically include some mechanism for breeding tritium from lithium.
For example, an international megaproject referred to as ITER is currently underway to engineer and build a nuclear fusion reactor that implements a D-T fusion reaction in a tokamak device configured to magnetically confine plasma in a toroidal chamber.
The ITER D-T fusion reactor system includes a tritium breeder blanket that performs two critical functions: (i) transforming the neutron energy generated by the D-T fusion into heat, and (ii) breeding tritium for tritium self-sufficient plasma operation.
Tritium is generated in the blanket by neutron transmutation reactions with lithium isotopes. The lithium in the blanket may be in liquid form, for example, as a eutectic alloy such as Li17Pb83, or in solid form, for example, a lithium-ceramic material sometimes called a solid breeder. The neutrons originate from the reactor plasma and enter the blanket with high energy (e.g., about 14.7 MeV). In practical energy systems, the D-T fuel cycle requires the breeding of tritium from lithium using one of the following reactions:n+36Li→T+∝+4.78 MeVn+37Li→T+∝+n−2.47 MeV
Where,
n=neutron,
T=tritium,
∝=helium ion.
The first reaction occurs at any energy and releases 4.78 MeV of kinetic energy per reaction. The second reaction is a threshold reaction and requires an incident neutron energy in excess of 2.47 MeV. A neutron multiplier, for example beryllium, may be used to increase the number of neutrons available to interact with lithium to produce tritium.
Current tritium breeder blankets use sphere pack or pebble bed technology, which imposes severe design and operational limitations. Pebble beds comprise densely packed micro spheres made of metallic or ceramic constituents.
However, conventional sphere-packed configurations impose stringent design and operational limitations on solid breeder blankets due to (1) a low effective thermal conductivity, (2) a maximum breeder packing fraction of ˜65%, (3) a relatively narrow operating temperature window (e.g., ˜325° C. to 925° C. for Li4SiO4), (4) uncertainty in pebble bed-wall interaction, particularly for cyclic operations, (5) fragmentation and failure of breeder pebbles due to high pebble contact stresses, (6) solid breeder pebble bed deformations due to thermal creep, swelling, cracking/fragmentation, and (7) marginal tritium breeding ratios, because temperature control of the breeder requires high structure-to-breeder volume fractions.
Tritium production and release characteristics are critical to the operation of a solid breeder blanket. At 400° C. the residence time of tritium can be as long as two hours in typical ceramic pebbles. At high Li burn-up, tritium release rates depend on the state of the interconnected pores and the size of the grain of the breeder material itself. The temperature gradient imposed in the breeder section can cause differential stresses and may lead to cracking and fragmentation of breeder pebbles, which can result in loss of breeder temperature control and may block the flow of tritium purge gas, leading to excess tritium trapping and unacceptable high levels of stagnant tritium inventory. A high tritium inventory in the blanket may become a threat to safety, and may require early removal of the blanket.
Because of its configuration and brittle nature, the thermo-mechanical behavior of the sphere-packed breeder bed represents key challenges for developing this line of blankets. This is due to, in part, that the tritium release characteristics and inventory in ceramic breeders strongly depend on temperatures. Operating ceramic breeder pebbles beyond the upper limit can induce sintering, which traps tritium leading to a huge tritium inventory. Furthermore, differential thermal expansion due to temperature gradients creates stress/strain conditions that affect the breeder effective thermal conductance and subsequent temperature distribution. These temperature-driven processes impose operating limits on the ceramic breeder region temperatures, and require acceptable accuracy in the prediction of the spatial and temporal temperature profiles over the lifetime of the blanket.
A novel alternative to pebble bed tritium breeder blankets is disclosed herein. In particular, a new class of solid breeder materials with an interconnected network of microchannels is disclosed. In these new materials, the internal network of interconnected microchannels provide large internal surface areas for efficient release of reactants, flow of reagents, catalytic reactions, or extraction of nuclear gaseous transmutation products, such as tritium in fusion reactor solid breeder materials or gaseous fission products produced in fuel rods in fission power reactors.
The materials disclosed herein have the following advantages: high reliability, standalone structure, network of interconnected microchannels for efficient release of tritium or flow of reagents or reaction products, high ceramic density, high reaction and/or tritium production rates, high tritium or other gaseous products release rates, high thermal conductivity, elimination of the large temperature drops between pebble bed and containment structure, elimination of pore closures due to sintering under operation, and elimination of pebble failure.